1. Field of the Invention
This invention relates to methods and apparatus for fluid catalytic cracking of hydrocarbon products with subsequent regeneration of catalyst particles, and more particularly, to methods and apparatus for fluid catalytic cracking employing a short contact time riser conversion zone, a separator for separating spent catalyst from a hydrocarbon stream, a high temperature stripper to control carbon level on spent catalyst, means for regenerating the catalyst after high temperature stripping, and means for returning regenerated catalyst to the riser conversion zone and the high temperature stripper.
2. Discussion of the Prior Art
The field of catalytic cracking has undergone progressive development since 1940. The trend of development of the fluid catalytic cracking process has been to all riser cracking, use of zeolite-containing catalysts and heat balanced operation.
Fluid catalytic conversion systems require a combined operation including separation of finely divided fluidizable catalyst particles from gasiform reaction products and regeneration of the catalyst employed therein by burning to remove deactivating carbonaceous deposits. Further, in present riser catalytic cracking operations, large amounts of catalyst are suspended in gasiform materials in the riser catalytic cracking units. It is necessary to separate rapidly the suspensions into a catalyst phase and a gasiform phase after the suspension conversion operation has traversed the riser unit or conversion zone. Various attempts have been made to provide improved suspension separation techniques to decrease losses in the catalyst phase or the gasiform phase resulting from overextending the conversion reactions.
Multistage stripping is already known in the prior art, as disclosed, for example, in U.S. Pat. No. 4,043,899 to Anderson et al. In addition, a catalyst terminating in an enclosed cylindrical vessel within a FCC reactor vessel and a riser containing baffles is disclosed by U.S. Pat. No. 4,206,174 to Heffley et al and risers attached to conduits are disclosed by U.S. Pat. No. 4,219,407 to Haddad et al.
Other major trends in fluid catalytic cracking processing have been modifications to the process to permit it to accommodate a wider range of feedstocks, in particular, stocks that contain more metals and sulfur than had previously been permitted in the feed to a fluid catalytic cracking unit.
Along with the development of process modifications and catalysts, which could accommodate these heavier, dirtier feeds, there has been a growing concern about the amount of sulfur contained in the feed that ended up as SO.sub.x in the regenerator flue gas. Higher sulfur levels in the feed, combined with a more complete regenerator, tended to increase the amount of SO.sub.x contained in the regenerator flue gas. Some attempts have been made to minimize the amount of SO.sub.x discharged to the atmosphere through the flue gas by providing agents to react with the SO.sub.x in the flue gas. These agents pass along with the regenerated catalyst back to the fluid catalytic cracking reactor, and then the reducing atmosphere releases the sulfur compounds as H.sub.2 S. Suitable agents for this purpose have been described in U.S. Pat Nos. 4,071,436 and 3,834,031. Use of a cerium oxide agent is shown in U.S. Pat. No. 4,001,375.
Unfortunately, the conditions in most fluid catalytic cracking regenerators are not the best for SO.sub.x adsorption. The high temperatures encountered in modern fluid catalytic cracking regenerators (up to 1600.degree. F.) tend to discourage SO.sub.x adsorption. One approach to overcome the problem of SO.sub.x in flue gas is to pass catalyst from a fluid catalytic cracking reactor to a long residence time steam stripper. After the long residence time steam stripping, the catalyst passes to the regenerator, as disclosed in U.S. Pat. No. 4,481,103 to Krambeck et al and incorporated herein by reference. However, the process described in U.S. Pat. No. 4,481,103 preferably steam strips spent catalyst at 932.degree. to 1022.degree. F. (500.degree.-550.degree. C.), which may not be sufficient to remove some undesirable sulfur- or hydrogen-containing components. Furthermore, catalyst passing from a fluid catalytic cracking stripper to a fluid catalytic cracking regenerator contains hydrogen-containing components, such as coke, adhering thereto. This causes hydrothermal degradation when the hydrogen reacts with oxygen in the regenerator to form water.
U.S. Pat. No. 4,336,160 to Dean et al attempts to reduce hydrothermal degradation by staged regeneration. However, in this process, the flue gas from both stages of regeneration contains SO.sub.x, which is difficult to clean.
It would be desirable to separate hydrogen from catalyst to eliminate hydrothermal degradation. It would be further advantageous to remove sulfur-containing compounds prior to regeneration to prevent SO.sub.x from passing into the regenerator flue gas.